Bone Anatomy: Compact & Trabecular Structure, Osteoblasts, Osteoclasts, Modeling, Study notes of Cell Biology

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Bone

Peter Takizawa

Department of Cell Biology

• Bone structure and composition
• Bone cells
• Bone modeling and remodeling
• Regulation of osteoclast activation

Mechanical bone consists of two architectures: compact and trabecular.

Compact bone

Trabecular bone

Kerr Atlas of Functional Histology 1st edition Most bone comes in two architectures, both of which are seen in this section of long bone. On the outer surface is compact bone that appears as a dense wall of bone but compact contains blood vessels and nerves. In the interior is trabecular bone that consists of network of bone struts. Trabecular bone reduces the weight of bone while still providing robust mechanical support. It also allows space for bone marrow and differentiation of blood cells. Because we are focusing on the mechanical functions of bone, it’s important to appreciate the structure and sections of long bone that are the primary load-bearing bones in the body. Long bones, as the name implies, are longer in one direction and consist of a shaft with two knob-like structures at either end. The shaft is called the diaphysis and the ends epiphysis. The outer portion of long bone is all compact bone and the inner portion is trabecular. The outer surface of the bone is called the periosteum and along the diaphysis is covered in a fibrous material. The epiphysis is covered with cartilage that cushions interactions with adjoining bones. The inner surface of compact bone is called endosteum or endocortical surface. The medullary cavity runs the length on the diaphysis and is the bone marrow is located. Long bones are similar to hollow tubes such as PVC pipe. PVC is remarkably good at resisting bending while being light due to its hollow core. PVC does not deal well with compression. To resist compression, bone utilizes trabeculae. The network of trabeculae can adsorbed a compressive force by bending. In long bones, the trabeculae are concentrated at the ends or epiphysis where the bone flattens somewhat to provide some cushion to compression. It’s in flat bones, such as vertebrae, where trabeculae function most prominently to resist compression.

Compact bone is organized into circumferential lamellae and Haversian systems. Compact bone consists of two arrangements of lamellar bone. The outer and inner surface are lamellae arranged circumferentially around the bone.These lamella circumnavigate the entire outer and inner surface of the bone. The second arrangement is called Haverisan systems or osteons. These are lamellae organized in a ring structure. In the center of the ring run blood vessels and nerves. The canals run parallel to the long axis of bone. A second canal system, Volkmann’s canals carries blood vessels into and out of compact bone.

The major structure components of bone are calcium crystals and type I collagen.

Ca

cyrstals

Collagen

Composition of bone Fibril Mineralized collagen fibril

The composition of bone is quite simple as it is made of two major components that interact to form a composite material: type I collagen fibrils and crystal of calcium-phosphate. Calcium-phosphate contains a mixture of calcium and phosphate and are often called hydroxyapatite. During bone formation hydroxyapatite crystallizes along the length of a collagen fibril to form a mineralized fibril that is the main structural component of bone. Bone also contains several minor protein components many of which facilitate the formation of calcium-phosphate crystals on collagen fibrils.

Bone can be arranged in lamellar or woven forms.

Lamellar

Woven

The mineralized collagen fibrils can be arranged in several different patterns but the two that are most prominent are lamellar and woven. In lamellar bone the fibrils are arranged in parallel arrays. This arrangement gives bone maximal mechanical strength and is the type you will see in most fully developed bone. In woven bone the fibrils are arranged in random orientations with little organization giving the macro structure of woven bone a much more disorganized appearance than lamellar bone. Woven bone is the immature form of bone and is seen in fetal development. It is also found at sites of fractures where the bone cells rapidly synthesize woven bone as a temporary stabilizing and replacement for lamellar bone. Eventually, woven bone is replaced by lamellar bone to provide better structural support.

Osteoblasts secrete collagen and catalyze the crystallization of calcium on collagen fibers. Osteoblasts are the cells that synthesize bone. Similar to fibroblasts they synthesize and secrete type I collagen that self-assembles into fibrils in outside the cell. Osteoblasts also secrete proteins that bind calcium and increase the amount of free phosphate, providing the raw materials for the formation of calcium- phosphate crystals. Over time the collagen fibrils will become coated with calcium-phosphate crystals to form bone.

Osteoblasts synthesize osteoid that mineralizes

into bone.

Osteoblasts that are actively synthesizing new bone are easy to identify in histological sections because of the presence of osteoid. Osteoid is unmineralized collagen fibrils and tends to stain pink in H&E samples. Calcified bone stains blue.

Osteocytes maintain connections with each other

through canaliculli and gap junctions.

Osteocytes

Within bone osteocytes maintain connections with each other through channels within bone called canaliculli. Osteocytes extend processes called filopodia through the canaliculli. Filopodia from adjacent cells contact each other and communicate via gap junctions.

Osteocytes maintain connections with each other

through canaliculli and gap junctions.

Within bone osteocytes maintain connections with each other through channels within bone called canaliculli. Osteocytes extend processes called filopodia through the canaliculli. Filopodia from adjacent cells contact each other and communicate via gap junctions.

Osteoclasts dissolve bone by secreting acid and collagenases.

Collagenases

The third cell type is osteoclasts. Osteoclasts digest and resorb bone. They accomplish this by secreting hydrogen ions to lower the pH and help dissolve the calcium-phosphate crystals. They also secrete proteases that digest collagen fibrils. Note the ruffled border of the plasma membrane that increases the surface area for secretion. One important feature of of osteoclasts is that they form a sealing border around the bone to prevent the loss of hydrogen ions and proteases to the surrounding environment. This increases the efficiency on bone resorption and prevents damage to surrounding material. The sealing border is generated by interactions between integrins in the plasma membrane of osteoclasts and fibers in bone.

Osteoclasts are multinucleated cells that scour the surface of bone. Osteoclasts are easy to identify because they are multinucleated and are found in regions where bone appears to be carved out.

Bone modeling and remodeling

Bone is a dynamic material that undergoes synthesis and resorption. Synthesis Synthesis Resorption Resorption Increase length or store Ca

Increase Ca

levels Maintain integrity One of the misconceptions about bone is that it is a static material. Bone is highly dynamic and is constantly being made and resorbed. Synthesis can due to the need to store excess calcium or increase the length of bone during development. Resorption can be triggered when blood calcium levels fall and calcium needs to be released from bone. The release and storing of calcium make minor changes in bone. Bone is constantly being resorbed and replaced by new bone. This is most apparent in load-bearing bones where damage to the integrity of bone can accumulate because of daily stress. This old, damaged bone is resorbed and replaced with new, undamaged bone. It’s similar to the streets that accumulate cracks and damage and have to be dug up and resurfaced on a continuous basis. What’s critical to the mechanical integrity of bone is to maintain a proper balance between resorption and synthesis. If the balance swings too far to one side, bone loses its mechanical integrity. Osteoporosis is thought to be a loss of balance in which resorption outpaces synthesis.